The value and need for visual acuity cannot be overstated. Historically, human beings have relied on eyesight as perhaps the most significant sensory perception employed in interacting with the environment. Unfortunately many persons, however, experience slight aberrations in the physical structure of the eye which aberrations interfere with the maximum acuity possible, These aberrations primarily manifest in three conditions: (1) myopia; (2) hyperopia; and (3) astigmatism.
Myopia is a condition wherein parallel light rays entering the eye are focused in front of the retina such that nearby objects are seen with greater clarity than distant objects. Myopia can be caused by an elongated eyeball or by a cornea which has too steep of a curvature resulting in greater refraction than desired. On the other hand, hyperopia is a condition wherein parallel light rays focus behind the retina so that distant objects are seen more clearly than nearby objects. Hyperopia results where the eyeball is foreshortened or where the curvature of the cornea is not steep enough for proper focusing of light rays onto the retina. Astigmatism is a defect of the eye whereby rays of light converge unequally in different meridians thus causing uneven focus or multiple images to be perceived. Astigmatism typically results where the cornea is slightly ovoid instead of circular.
Until the middle ages, people were generally confined to the visual acuity provided by their respective eye physiology. However, greater understanding of optics lead to the creation of corrective lenses which, when placed in front of the eye, could compensate for visual defects thereby causing a redirection of light whereby the defective eye would focus light onto the retina. As the understanding of both optics and eye physiology grew, lenses became more and more sophisticated. In the present day, lenses can correct for myopia, hyperopia, astigmatism and other eye conditions. Lenses are sometimes designed to have different focusing properties in different sections, such as bifocals and trifocals, in order to compensate for different visual acuity of an eye at different distances.
While the invention of eyeglasses may well be one of the most significant contributions to the quality of human life, at least for those who have visual defects, eyeglasses have their drawbacks. For example, eyeglasses provide a generally conic field of vision defined by that volume which is the geometric projection of the perimeter of the lens from the focal point thereof; thus, there is no enhancement of peripheral vision. Moreover, many people find the wearing of eyeglasses to be somewhat uncomfortable, especially if they are not fit properly. Discomfort can result from the weight of the lenses as well as the frame holding them or an improper fit of the frame to the differing shapes of the human head. During times of high activity wherein the need for visual acuity is often greatest, eyeglasses can tend to become dislodged resulting in a loss of visual acuity at a time when it is especially desired. Finally, some persons perceive the wearing of glasses to be aesthetically unpleasing. Unfortunately, the wearer of eyeglasses may sometimes be self-conscious or openly subjected to derisive comments from others.
As a result of the disadvantages of eyeglasses, efforts were made to create lenses which would fit directly onto the eye which would allow greater peripheral vision, which would be more secure and which would be less noticeable to others. Such a lens, commonly referred to as a "contact lens", had their first major development circa 1945. At the time of inception, these lenses were large shells which fit over substantially the entire exposed surface of the eyeball. These early lenses caused substantial irritation to the eye and to the surrounding tissues. Accordingly, they could only be worn for a short period of time.
Over the next two decades, the physical size of contact lenses decreased, and improvements were made in materials technology so that contact lenses came to be made of plastic and were of reduced thickness and diameter. The diameter of the lens was reduced to approximately 8 to 10 millimeters, corresponding to the necessary corrective dimensions needed for the eye over differing light conditions. While these hard plastic lenses were substantial improvements over the earlier glass lenses, they nevertheless still caused irritation and sometimes pain to the wearer. In part this was due to the irritation of the eye and inner surface of the eye lid but it also was a function of diminished oxygen exchange by the lubricating fluids of the eye which were trapped between the eyeball and the lens.
Another avenue investigated for correcting defects in visual acuity has been through eye surgery. Surgery for correcting certain defects in the eye, such as vision obscuring cataracts, dates back approximately two thousand years wherein a needle was inserted into the eye in a manner to dislodge the occluded lens and push it out of the optical path. It was not until the 1960's, however, that significant developments towards surgical correction of the refractive properties of the eye became developed.
One technique pioneered at this time has come to be known as radial keratotomy which can be used to correct mild levels of myopia. In radial keratotomy, the cornea of the eye is slit along a plurality of lines radiating from the pupil with such cuts being made at constant depths. The result is that the cornea relaxes to reduce the steepness of its curvature thereby resulting in a longer focal length. The overall result is that the focal point of the eye is moved rearwardly onto the retina. Radial keratotomy, however, is not available as a treatment for hyperopia since hyperopia requires the creation of a greater lensing effect. Similar in concept to radial keratotomy is astigmatic keratotomy wherein arcuate slits are selectively cut into the eye at desired radial distances so that the ovoid configuration of the cornea becomes more round.
Somewhat contemporaneously with the development of radial keratotomy was the investigation into a technique that became known as, lamellar refractive surgery. A first developed technique was myopic keratomileusis (MKM). Here, a surgeon would cut a dome-like shell or cap off of the cornea and remove a disk of corneal tissue after which the cap was replaced. The removal of the cornea tissue reduced the steepness of the corneal curvature thereby lessening its lensing effect resulting in correction of a myopic condition. This technique had three main drawbacks. First, proper rotational orientation of the corneal cap on replacement was difficult. Second, even after the repositioning of the corneal cap, it was subject to dislodgement and possible loss during the healing process. Third, and significantly, it was difficult for a surgeon to manually cut a uniform disk out of the cornea. Any anomalies in the thickness of the removed disk created non-uniform correction.
Two major advances helped establish MKM as a viable corrective technique. First, the rotational issue of the corneal cap was resolved by developing a technique wherein the dome-like cap was not cut completely off of the cornea but was rather left attached by a small hinge of tissue along one edge. During surgery, this cap was then pivoted out of the way while the corrective tissue disk was removed from the cornea after which the cap was repositioned. The tissue hinge maintained proper orientation and also helped decrease the likelihood of dislodgement and loss of the corneal cap during the healing process. Second, advances in computer technology and better instrumentation resulted in the development of the automated microkeratome. This device, in essence, is an automated scalpel which could be computer controlled to cut a fairly uniform thickness disk out of the cornea after the corneal cap had been cut and pivoted out of position. This could be accomplished directly on the globe portion of the eye and therefore avoid the necessity of freezing the corneal cap as had been the case with MKM. This operative procedure came to be known as automated lamellar keratoplasty (ALK) and its use became more accepted in the late 1980's. This technique was advantageous for its ability to correct more extreme cases of myopia.
Next in the development of surgical techniques for corrective eye surgery was the laser technique known as the excimer laser technique. In this technique, the surface of the cornea is burnt away in a desired configuration by a light intensity laser beam operating at 193 micron wavelength. The laser beam is controlled in intensity and pattern to etch away surface corneal tissue so as to alter its curvature and thereby its refractive properties in order to correct myopic and astigmatic conditions.
The excimer laser is a somewhat painful process, though, since it burns away the epithelia layer which requires one to two weeks to heal. In addition, the excimer laser sometimes causes undesired scarring of the cornea. The excimer laser, however, does show substantial promise for correcting mild to medium myopia and astigmatism.
Recently, however, perhaps the most promising of all surgical techniques has been explored using a combination of automated lamellar keratoplasty and the benefits of excimer-type lasers. This procedure, which is known as laser assisted intrastromal keratomileusis (LASIK) involves the cutting of the dome-like corneal cap utilizing the microkeratome. After cutting the cap, the microkeratome is removed from the support assembly and the cap is pivoted out of position. Laser focusing optics are then brought into position over the exposed, domed surface of the cornea formed by removal of the cap shell. The laser light is then very precisely controlled so that a desired contoured shaping of the exposed surface occurs by varying the tracking, intensity and size of the laser beam. After performing the sculpting of the corneal interior, the cap is repositioned and the cornea reheals with a modified curvature.
LASIK surgery is especially promising in that it provides numerous advantages over previous surgical techniques. On one hand, the use of a laser surgical technique can be very precisely controlled, as is the case with traditional excimer surgery, so that the cutting of the proper corrective contour can be very precise. In addition, since the removal of tissue occurs in the inside of the cornea, the epithelial layer and the Bowman's layer are not removed so that very little pain and less damage to the cornea occurs. The ability to control the laser also allows for the simultaneous correction of both the necessary refractive prescription as well as correction of astigmatism. The LASIK technique also has the benefit of being able to correct severe degrees of myopia up to approximately 30 diopters. Finally, unlike many of the other techniques, the LASIK procedure holds great promise for its ability to correct hyperopia as well as myopia.
One issue of concern in LASIK, as well as in ALK, procedures is the manner in which the dome-like corneal cap is repositioned onto the cornea after the corneal-sculpting procedure is performed. Particles such as dust from the air or loose tissue particles remaining after the cornea is sculpted must be removed from between the cornea and the corneal flap, such as by irrigation, after repositioning the flap onto the eye. Also, the flap must be drawn across the eye so as to make a seal and prevent wrinkling of the corneal flap.
A device currently used in the repositioning of the flap is called a cannula. The cannula generally is a single-use or multi-use needle-like structure which can be attached to a syringe. The syringe can hold fluid useful in irrigating the eye to remove particles after repositioning the flap onto the cornea. Generally, the cannula is a tubular structure open at both ends so that fluid may flow from the syringe through the cannula and onto the eye. One end portion of the cannula is connectably attached to the syringe by an engaging mechanism. The other end portion is open at the distal end. The cannula may be a straight tube or may be bent so as to extend at an angle from the longitudinal direction of the syringe.
After the flap is returned to the cornea, the cannula is inserted between the cornea and the flap. The syringe plunger is depressed so as to force fluid through the cannula onto the eye, thus irrigating the interface between the cornea and the flap and causing the removal of any particles trapped in the interface. The cannula is then drawn across the cornea so as to squeeze any excess fluid out of the interface and to draw the flap into proper position.
Many problems exist regarding the use of cannulas of the structure described above. Difficulties are encountered with the attempted insertion of the cannula underneath the flap and into the interface between the cornea and the flap. The distal end of the cannula may be sharp enough to cause poking of the eye resulting in damage to the eye. Fibers in the interface between the flap and the cornea may catch on the sharp end of the cannula resulting in further damage to the eye. Further, fluid may flow onto the eye from the distal end of the cannula at a rate and pressure greater than that desired.
In any event, it can be appreciated that there remains a need for an improved device for irrigating and repositioning the cornea flap onto the cornea after corneal-sculpturing has been performed in corrective eye surgery procedures such as LASIK and ALK. The present invention is directed toward such need and this invention concerns a device for irrigating and repositioning a corneal flap onto a cornea after various methods of corrective eye surgery.